The present invention relates to electronic reproduction techniques and to the production of printing blocks.
In electronic reproduction apparatus for producing printing blocks or plates, an original is scanned by dot-by-dot and line-by-line by means of an opto-electronic scanning element and the image signal obtained by doing so is corrected according to the requirements of the reproduction process. The corrected image signal controls a recording element which records the screen-based or screenless printing block on to a recording medium, if appropriate to a reproduction scale altered as compared to the original scanned.
A diminution of contrast perceived as blurring by the human eye occurs during the production of originals for scanning and due to the limited capacity of resolution of the opto-electronic scanning element.
U.S. Pat. No. 2,691,696 discloses a method of enhancing contrast or definition, which corresponds to diffusion masking in photographic reproduction technique. A "surround" signal is obtained in this known method in addition to the image signal, by also scanning the "surround" or surrounding area by means of a commensurately larger surround aperture member apart from scanning the actual image dot by means of an image dot aperture member, forming the differential signal from the surround and image signals and superimposing the same on the image signal in selectable strength as a correction signal for contrast or definition, the effective amplitude of the correction signal being determined by the diameter of the surround aperture member utilised. The increase of contrast or definition is accomplished by the fact that, within the effective amplitude of the correction signal, a dark image portion is recorded darker, and a light image portion lighter, than outside the effective amplitude.
In most instances, the originals which are to be reproduced consist of half-tone images having diffuse outlines, combined with characters or line elements of graphic illustrations having sharply defined outlines.
Since, apart from the preset degree of amplification, the amplitude of the correction signal also depends on the magnitude of a tonal value or image signal change, the contrast is normally accentuated in substantially higher degree on line elements and character outlines (large image signal change) than for outlines in half-tone images (small image signal change). It is however precisely the object in practice to accentuate the detail contrast in half-tone images, since character outlines and line elements are reproduced with sharper definition in any event. In order that the required effect of a contrast accentuation may be secured at all in half-tone images, the available correction signal should operate at full strength. Troublesome white borders or edges which are extremely noticeable, e.g. if black type appears in a grey shade, are formed however in this case on highlights, sharp outlines and around black characters. These borders or edges are particularly troublesome however if the printing block is recorded to a scale larger than the original, since the effective amplitude of contrast accentuation on the recording medium varies with the reproduction scale.
United Kingdom Patent Specification No. 1,546,359 discloses a reproduction device comprising a system for accentuation of contrast or definition in which, for the purpose of adapting the scanned line width to the reproduction scale, the opening of the image dot aperture member are varied as a function of the reproduction scale and the aperture of the surround aperture is also varied at the same time by means of mechanically coupled diaphragm plates, in such manner that the ratio between the diameters of the image dot and surround shutters remains constant. Although these measures may well alter the effective range of the contrast accentuation as a function of the image dot aperture member utilised or of the reproduction scale to the effect of a reduction of the troublesome edges or borders, the known reproduction device still has the disadvantage that the effective range cannot be optimised for special reproduction requirements.